1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus and, more particularly, to a high-speed reproducing system, a tracking control system and a tension control system for a video tape recorder (hereinunder referred to as "VTR") of a helical scanning system.
2. Description of the Related Art
In a conventional auto tracking reproducing apparatus in a VTR of a helical scanning system, a video signal reproducing magnetic head is generally mounted on an electromechanical transducing element (hereinunder referred to as "head actuator"). At the time of reproduction, the head actuator is driven perpendicularly to the direction of the travel of a recording track, thereby giving automatic following control over the magnetic head so as to prevent the magnetic head from leaving the recording track.
Various methods have been proposed and already put to practical use concerning a technique of enabling the magnetic head mounted on the head actuator to automatically follow the recording track, which is called an auto tracking control technique.
For example, several kinds (e.g., four kinds) of pilot signals for tracking low frequencies in a band other than a video signal band are overlapped with a video signal and separate pilot signals are recorded on several adjacent (four) tracks, as known in an 8-mm VTR format. In this pilot system, a tracking error signal is detected by a difference in a crosstalk level between the left and right tracks at the time of reproduction.
In a wobbling system which has been put to practical use in 1-inch VTR produced by Ampex, D-2 format digital VTR DVR-10 produced by Sony Corporation and the like, the magnetic head is forcibly vibrated minutely in the direction of the track width at a constant frequency which is called a wobbling frequency. The reproduction envelope signal from the magnetic head is synchronously detected at the wobbling frequency, whereby a tracking error signal is detected.
In a mountaineering system, which has been put to practical use in VHS VTR, NV-10000 produced by Matsushita Electric Industrial Co., Ltd., VHS VTR F75 produced by Mitsubishi Electric Copr. and the like, the reproduction envelope signal from the magnetic head is supplied to a sample-and-hold circuit at the central portion of the field which has been read. The voltage to be applied to the actuator or the rotation phase of the capstan motor is then changed by one step (e.g., increased) and the envelope level of the next frame is compared with the value of the sample and hold value. This series of operations is repeated until the envelope level of the next frame becomes smaller. When the envelope level of the next frame becomes smaller, the direction of the applied voltage is reversed and the envelope level of the reproduction signal is upwardly converged toward the maximum value.
In a conventional auto tracking reproducing apparatus, a tracking error is detected by the above-described various tracking error detecting methods and the detected tracking error is fed back by the head actuator which is accommodated in the rotary drum.
Such a movable head is not only used for dynamic track following (hereinunder referred to as "DTF") control for correcting a tracking error during normal-speed reproduction but also often used at the time of superior reproduction (high-speed reproduction, slow reproduction and still reproduction).
As an example of such a movable head used for noiseless superior reproduction, the system of a movable head described on p. 41 of National Technical Report. Vol. 28, No. Jun. 3, (1982) is schematically shown in FIG. 75.
In order to briefly explain the high-speed superior reproducing method using this known system, FIG. 76 shows the high-speed superior reproduction servo system. In FIG. 76, a rotary magnetic head 1 is driven by a head actuator 2 perpendicularly to the direction of the travel of the tape. From the reproduction envelope signal of the magnetic head 1, an amount of tracking error is detected by a tracking error detector 3 and a tracking error signal is output. An inclination correction pattern generator 4 corrects the inclination of the magnetic head 1 from the tape speed information so that the angle at which the magnetic head 1 scans a tape (not shown) agrees with the angle of the recording track (not shown) and generates a track trace pattern for the magnetic head 1. The tracking error signal from the tracking error detector 3 and the inclination correction pattern from the inclination correction pattern generator 4 are added by an adder 5.
The operation of a conventional system will now be explained. The angle at which the magnetic head 1 traces the tape at the time of normal-speed reproduction (hereinunder referred to as "speed 1") is the same as the angle of the recording track. However, at the time of reproduction at a different speed, since the angle at which the magnetic head 1 traces the tape does not agree with the angle of the recording track, off-track (hereinunder referred to as "inclination error") is produced and a noise is produced on a reproduced picture.
As an example, FIGS. 77 and 78 schematically show the relationship between the recording track pattern on the tape and the trajectories of the magnetic head 1 produced at the time of reproduction at a speed five times as high as the normal speed (hereinunder referred to as "speed 5") in the forward direction and the reverse direction, respectively. In FIGS. 77 and 78, the symbol A represents a trajectory of the magnetic head 1 produced at the time of normal-speed reproduction, B a trajectory of the magnetic head 1 produced at the time of reproduction at speed 5, and C a trajectory of the magnetic head 1 produced at the time of reproduction at speed 5 in the reverse direction. As is obvious from FIGS. 77 and 78, the trajectory of the magnetic head 1 must be corrected to be A from B or C for the purpose of noiseless reproduction.
FIG. 79 schematically shows an inclination error pattern of the magnetic head 1 produced at the time of reproduction at a speed n times as high as the normal speed (hereinunder referred to as "speed n", n is any given real number).
It is now assumed that a VTR has a guard bandless recording system utilizing an azimuth loss. If it is assumed that T is a 1/2 period of the rotary drum and t.sub.p is a track pitch, the inclination error which may be caused at the time of reproduction at speed n is represented by t.sub.p (n-1), wherein n is an integer. In this way, the inclination error pattern is represented by a function having n as a parameter. In other words, the inclination error pattern changes depending upon the tape travelling speed. The inclination correction pattern generator shown in FIG. 76 is so designed as to generate an inclination correction pattern by utilizing tape speed information such as a capstan FG signal.
When the inclination correction pattern is supplied to the head actuator 2, the inclination of the magnetic head 1 is corrected so as to move in parallel to the trajectory of the recording track even at the time of reproduction at a different speed. However, the mere displacement of the magnetic head 1 in conformity with the angle of the recording track further generates off-track due to the linearity of the trajectories of the recording track and the magnetic head 1 or the phase deviation of the track. In order to prevent such off-track, an auto tracking control system by a closed loop, which is represented by the surrounding broken line in FIG. 75, is generally added.
Any system such as the pilot system, wobbling system and mountaineering system described above may be adopted as the controlling method for the auto tracking control system, but in order to obtain a high-definition image even at the time of production at a different speed, since it is necessary that the magnetic head 1 follows the nonlinearity of the recording track (hereinunder referred to as "track rolling"), it is desirable to adopt the pilot system or the wobbling system which allows a comparatively wide controlled region. Since the controlling method and operation of the auto tracking control system have already been known, a detailed explanation thereof will be omitted here.
The tape tension control will now be explained.
FIG. 80 shows the structure of a magnetic tape travelling system of a video tape recorder of a VHS system as a magnetic recording and reproducing apparatus which is described on p. 187 of Introduction to Magnetic Recording Technique, by Yokoyama, edited by Sogo Denshi Shuppan-sha. In FIG. 80, a video tape (magnetic tape) is supplied from a feed reel 6 and the tension of the magnetic tape travelling system is detected by a back tension post 7. The information recorded on the video tape is temporarily erased by an all-width erase head 8. The magnetic tape travelling system is stabilized by impedance rollers 9 and 10. A rotary drum 11 includes an upper cylinder 12 and a lower cylinder 13. A video head 14 is secured to the upper cylinder 12. The sound signal on the linear track of the video tape is erased by a sound erase head 15, and thereafter a sound and a control pulse are recorded on the linear track by a sound control head 16. A pinch roller 18 is provided so as to clamp a capstan shaft 17 and a video tape at a constant pressing force. The capstan shaft 17 is opposed to the pinch roller 18 so as to control the deviation of the trajectory of the video head 14 from the video track on the video tape by causing the video tape to travel. A take-up reel 19 is provided for taking up the video tape.
FIG. 81 shows the structure of a conventional tension control mechanism (tension servo mechanism). In FIG. 81, the rotation of the feed reel 6 is suppressed by a hub brake 20. The tension of the magnetic tape travelling system is detected by a tension control arm 21. The force proportional to the amount of displacement of the tension control arm 21 is applied to the hub brake 20 by a spring 22, the spring 22 being capable of varying the force which is applied to the tension control arm 21. A tension adjust lever 23 for adjusting the reference tension of the tension control mechanism is connected to the spring 22.
The operation of the conventional tension control mechanism will now be explained.
The video tape supplied from the feed reel 6 is clamped between the pinch roller 18 and the capstan shaft 17 and stretched by the rotation of the capstan shaft 17. Thereafter, the video tape is wound around the take-up reel 19. During this time, it is necessary that the tension of the magnetic tape travelling system is controlled to a constant value so that the spaces between the video tape and the all-width erase head 8, the video head 14, the sound erase head 15, the sound control head 16 and the like are optimum. Needless to say, when the tension of the travelling system is increased, the spaces between the heads and the tape are reduced, so that the high-frequency characteristics of the recording and reproducing system are enhanced but the scuffs of the tape are increased and the durability of the apparatus in the still state for continuously reproducing the same track is deteriorated. In addition, the wear of the heads are increased. On the other hand, if the tension of the travelling system is reduced, since the spaces between the heads and the tape are increased, the high-frequency characteristics of the recording and reproducing system are deteriorated.
As a countermeasure, a conventional VTR is provided with a tension control mechanism such as that shown in FIG. 81. In FIG. 81, for example, if the tension of the magnetic tape travelling system is increased, since the balance between the tension control arm 21 and the spring 22 is disturbed, the spring 22 is extended. At this time, the hub brake 20 is relaxed and the rotation of the feed reel 6 is made free, whereby the amount of feed of video tape is increased. As a result, the tension of the magnetic tape travelling system is restored to the original tension. In this way, the tension of the magnetic tape travelling system is kept constant.
In a high-definition TV or a digital VTR for digitally recording and reproducing a video signal and a sound signal, since the amount of information recorded is greatly increased, the technique of high-density recording and reproduction with high-accuracy DTF control are essential in order to enable long-time recording on a cassette tape of a limited size.
In a DTF apparatus in a conventional VTR, since the means for tracking error correction is merely a movable head accommodated in the drum, the DTF control capacity is determined by the performance of the head actuator for moving the movable head.
As the head actuator 2 which is generally used for DTF control in a wide frequency band with a high accuracy, one which has no phase shift up to a comparatively high frequency, for example, up to the vicinity of 1 KHz to several KHz is selected by virtue of its good control capacity. The head actuator 2 which does not cause a phase shift up to a high frequency is required to have a mechanical characteristic which does not resonate up to a high frequency. The primary mechanical resonance frequency of a general actuator is obtained by dividing the root of the quotient obtained by dividing the spring constant of the actuator by the mass of the movable portion of the actuator by 2.pi.. A high primary resonance frequency is therefore obtained either by lightening the mass of the movable portion of the actuator or by increasing the spring constant of the actuator.
As described above, a movable head is generally not only used for DTF control at the time of normal-speed reproduction but is also often used at the time of superior reproduction. In a high-speed noiseless reproducing apparatus in a conventional VTR, the tracking error is corrected by moving the magnetic head in the direction of the width of the recording track by the head actuator. The amount of tracking error which is correctable is therefore limited to the range in which the head actuator is movable. For this reason, the range in which the head actuator for driving the magnetic head is movable is preferably as wide as possible. In a conventional structure, however, the head actuator must be accommodated in the rotary drum, the outer diameter of which is determined by the standard, so that a small-sized head actuator is naturally required.
To meet such demand, a piezoelectric element consisting of two pasted piezoelectric sheets (hereinunder referred to as "bimorphous cell"), a lamination type piezoelectric element with a displacement enlarging mechanism, such as a lever and a buckling spring attached thereto, and a moving coil supported by a spring and electromagnetically driven in a magnetic circuit (hereinunder referred to as "electromagnetic actuator") have been proposed as a small-sized head actuator which has a wide movable range, and some of these have been put to practical use.
The cases of using these head actuators for DTF control and high-speed noiseless reproduction will be considered in the following discussion.
It is first assumed that a bimorphous cell is used as a head actuator. A bimorphous cell is known among piezoelectric elements as an element which has a large amplitude for the driving voltage. The amount of displacement .xi. of a bimorphous cell is represented by the following equation: ##EQU1## wherein .xi.: displacement, V: applied voltage, d31: piezoelectric constant, l: effective length, t: thickness of one sheet of piezoelectric element, S.sub.k : electrode coefficient (0.94 to 0.95), R: loss factor (0.9)
The piezoelectric constant d31 is a function of the applied voltage V, and when the applied voltage V is large, d.sub.31 also becomes large. S.sub.k and R are constants determined by the configuration of the bimorphous cell.
Thus, it is understood that the amount of displacement .xi. of the bimorphous cell is determined by various factors.
In order to increase the primary mechanical resonance frequency of a bimorphous cell for DTF control, it is necessary to increase the thickness t of one sheet of piezoelectric and reduce the effective length l. In other words, it is necessary to reduce l/t. However, if l/t is reduced, the amount of displacement .xi. of the bimorphous cell is also reduced by the square of l/t, which is disadvantageous to the bimorphous cell for high-speed superior reproduction which requires a large amplitude. That is, a bimorphous cell for DTF control and a bimorphous cell for high speed superior reproduction have antipodal requirements. In most cases, the system of a bimorphous cell is therefore composed with more importance attached to either DTF control or high-speed superior reproduction.
For example, in a tape format having a wide track pitch such as the tapes of a publicly used VTR of VHS system and .beta. system and an 8-mm tape, since DTF control with a comparative accuracy is not required, the head actuator is mainly used for high-speed superior reproduction, as in known systems.
In this case, a head actuator having a large piezoelectric constant d.sub.31 is selected so as to have a large amplitude and a small mechanical resonance gain. However, it is the effective length l of the bimorphous cell in the term of a square that mainly influences the amount of displacement .xi., and the larger the effective length l, the larger the amount of displacement .xi..
As the head actuator is accommodated in the rotary drum having a limited diameter, as described above, the effective length l is also limited. Various attempts have been made at increasing the effective length l as much as possible. For example, there are an annular bimorphous cell 2a and carrying heads 14a and 14b shown in FIG. 82, which is disclosed in Japanese Patent Laid-Open No. 22285/1980 and leaf bimorphous cells 2b and 2c shown in FIG. 83, which are disclosed in Japanese Patent Publication No. 41130/1988. However, even if the amount of displacement .xi. is increased by increasing the effective length l in this way, there remains still another problem.
FIG. 84 shows the relationship between the effective length of a bimorphous cell and the inclination of a magnetic head. As is clear from FIG. 84, a large amplitude increases the inclination of the magnetic head, which inevitably results in the deterioration in the picture quality.
On the other hand, in a tape format having a narrow track pitch such as the tapes of a high-definition TV VTR and a digital VTR, since DTF control with a high accuracy in a wide frequency band is essential, a bimorphous cell having a high primary mechanical resonance frequency is selected even at the sacrifice of the possible speed in high-speed superior reproduction.
As described above, it is impossible that a bimorphous cell simultaneously satisfies both requirements for DTF control in a wide frequency band with a high accuracy and for high-speed superior reproduction.
Secondly, it is assumed that a lamination type piezoelectric element with a displacement enlarging mechanism attached thereto is used as a head actuator. An example of this type of head actuator is described in NEC Technical Reports, Vol. 40, No. 5. pp. 118 to 122 (1987). In this example, no inclination of the head is caused by displacement unlike a bimorphous cell, but since a lamination type piezoelectric element having a small amount of displacement is used as a driving element, it is impossible to obtain a large amount of displacement. Even if the amount of displacement is largely increased by a level or a buckling spring, when the head actuator is accommodated in the rotary drum of the VTR, the displacement is influenced by the centrifugal force of the displacement enlarging mechanism, thereby causing an offset in the displacement.
Thirdly, it is assumed that an electromagnetic actuator is used as a head actuator. An example of the electromagnetic actuator is disclosed in Japanese Patent Laid-Open No. 173219/1988. An electromagnetic actuator is known to have a comparatively large amount of displacement in comparison with the above-described two actuators. The structure of an electromagnetic actuator is shown in FIG. 85.
In FIG. 85, the head 14 is held by a movable coil 24 and the movable coil 24 is supported around a permanent magnet 25 in such a manner as to be movable in the axial direction. The position of the head 14 is therefore adjustable as desired by supplying an appropriate driving current to the movable coil 24.
Such an electromagnetic actuator has many advantages when it is used for high-speed superior reproduction. For example, a driving voltage V of several volts is sufficient, and there is no hysteresis or no inclination of the head. The high reliability is secured. There is no deterioration with time. In addition, since an electromagnetic actuator is cheap, it is suitable to practical use for a publicly used VTR. However, a general electromagnetic actuator for superior reproduction has a frequency response characteristic such as shown in FIG. 86. When an electromagnetic actuator is used for superior reproduction, since the spring constant is set at a weak value with respect to the force generated by the coil in order to increase the amount of displacement, the mechanical resonance frequency is low. In addition, it is necessary to sufficiently separate the driving coil from the magnetic head through a certain member in order to avoid the influence of the magnetic field generated from the driving coil while driving the actuator. Since the secondary resonance frequency caused by this member exists comparatively close to the first resonance frequency, the DTF control system must be composed by a compensation outside the resonance for controlling in a low frequency band than the primary resonance frequency. That is, since it is impossible to enlarge the controlled region, a DTF control in a wide frequency band with a high accuracy is not realized.
If the spring constant is increased for DTF control, DTF control in a wide frequency band with a high accuracy is possible. However, in order to use such an actuator for superior reproduction, it is necessary to increase the force generated by the driving coil for the purpose-of obtaining a displacement with a large amplitude. It is therefore necessary to apply a large current, which is a problem in the respect of heat generation or the like.
Consequently, it is also difficult that an electromagnetic actuator has a DTF control capacity and a high-speed superior reproducing capacity at the same time.
To sum up the above explanation, it is impossible in a conventional apparatus to simultaneously realize DTF control in a wide controlled region such as several hundred Hz with a high accuracy and noiseless reproduction at a high speed such as several ten times as high as the normal speed.
The problems of a tension control device in a conventional magnetic recording and reproducing apparatus will now be explained.
FIGS. 87 and 88 show a conventional magnetic recording and reproducing apparatuse, and in particular, a tape tension control device described in Japanese Patent Laid-Open No. 56036/1990. FIG. 87 shows a recording and reproducing state, and FIG. 88 shows a high-speed tape travelling state.
A magnetic tape is drawn out of a tape cassette (not shown) and constitutes a tape travel path such as that shown in FIG. 87. A tension lever 28, and arms 29 and 30 are integrally rotatable around a hinged support 31.
At the time of recording and reproduction, a tension post 33 is brought into contact with the magnetic tape and simultaneously a tension band 34 is brought into contact with a feed reel 35 by a slider 32, as shown in FIG. 87. The magnetic tape is fed toward a take-up reel 36 by a capstan at a constant speed and supplied from the feed reel 35. At this time, the moment of the tension lever 28 produced by a spring 38 between a tension release lever 37 and the tension lever 28 is balanced with the resultant force of the moment of the tension lever 28 produced by the force applied to the tension post 33 by the tension between tape guides 39 and 40 and the moment produced by the frictional force between the tension band 34 and the feed reel 35. The tension of the magnetic tape is mainly controlled by the frictional force applied to the feel reel 35 by the tension band 34.
For example, if the tape tension becomes larger than the balanced value due to an external disturbance, the tape tension between the tape guide 39 on the feeding side and the tape guide 40 on the take-up side as seen from the tension post 33 also becomes large. As a result, the tension post 33 is pushed out to the left-hand side of the balanced position shown in FIG. 87. The tension lever 28 is thereby rotated counterclockwise around the hinged support 31 and simultaneously the arm 30 is also rotated counterclockwise. With the reduction in the contact force between the tension band 34 and the feed reel 35, the frictional force is reduced, and consequently the tension is relaxed, whereby the tension post 33 is restored to the balanced position in the end.
On the other hand, when the tape tension becomes smaller than the balanced value due to an external disturbance, the frictional force between the tension band 33 and the feed reel 35 becomes large and, as a result, the tension is increased, whereby the tension post 33 is restored to the balanced position.
The tape tension is kept constant in this way at the time of recording and reproduction.
During high-speed tape travel, the tension post 33 is moved to a position at which the tension post 33 is out of contact with the magnetic tape by the slider 32. The tension band 32 is relaxed to a position at which the tension band 33 is out of contact with the feed reel 35, so that the tension control mechanism is separated from the tape travelling system. The tension control mechanism is also separated from a capstan 41 and a pinch roller 42. In the case of high-speed tape travel from the feed reel 35 to the take-up reel 36, the take-up reel 36 is rotated at a desired speed to wind the magnetic tape therearound, and a constant load is applied to the feed reel 35 to an extent which prevents the relaxation of the magnetic tape. On the other hand, in the case of high-speed tape travel from the take-up reel 36 to the feed reel 35, the feed reel 35 is rotated at a desired speed to wind the magnetic tape therearound, and a constant load is applied to the take-up reel 35 to an extent which prevents the relaxation of the magnetic tape.
In the tape tension control mechanism of a conventional magnetic recording and reproducing apparatus having the above-described structure, a special tape tension control other than the application of a load in the direction contrary to the direction of the travel of the tape is not exerted during high-speed tape travel. Therefore, the conventional tape tension control mechanism cannot respond to a transient tension change, and the tape is sometimes damaged, or by a change in the contacting state between the magnetic head and the tape caused by a tension change, a fluctuation in the output which leads to deterioration in information is apt to be caused.
In addition, the tension controlled region for the conventional tension control device is narrow, and a tension change which can be suppressed by the conventional tension control device is only not more than several Hz. Therefore, in a VTR for high-density recording and reproduction such as a digital VTR adopting this tape tension control device, it is impossible to constantly keep the optimum space between the magnetic head and the magnetic tape, thereby making good recording and reproduction impossible.